1. Field of the Invention
The present invention concerns a superconducting coil, in particular for a magnetic resonance apparatus, formed by one or more wound, superconducting coil conductors that are embedded in a cured sealing compound.
2. Description of the Prior Art
In magnetic resonance apparatuses diverse coils are used that—as basic components of such a magnetic resonance apparatus—serve for the generation of magnetic fields. A central coil is a superconducting coil (also frequently called a “magnet coil”) that is composed of multiple layers of wound coil conductors made of superconducting material with an essentially rectangular cross section (which is different than, for example, the gradient coil with its wound coil conductors made of copper), wherein the conductors are wound as tightly as possible in order to achieve an optimally high packing density. Such superconducting magnet coils are operated below the critical temperature of the superconducting material; they are cooled to extremely low temperatures before being fed with current. Conventional superconductors are cooled by means of liquid helium—the temperature is 4.2 K; high-temperature superconductors are cooled with liquid nitrogen to a temperature of approximately 77.4 K. The coil conductors themselves are cast in a hardened (cured) sealing compound, typically based on epoxide/amine or epoxide/acid anhydride; they are consequently completely embedded in the sealing compound. For fixing during the winding, a coil carrier is provided on which they are wound. Due to the extreme temperature differences or the extreme cooling and the different coefficients of thermal expansion of the materials involved (coil conductor (superconductor), epoxy resin, coil carrier), there are inevitably large mechanical stresses. These also build during operation, thus when the superconducting coil conductors are fed with current, since Lorentz forces can be produced that by the generated magnetic fields can act on the coil with respect to the coil.
In designing the structure of the coil, great care must be taken and it must be prevented without exception that a coil conductor within a magnet winding can move under an external force effect (for example the mechanical stress upon cooling) or due to Lorentz forces upon being fed with current. A conductor movement inevitably leads to the creation of heat as a result of the transduction of kinetic energy of the moved conductor or its deformation. The arising heat could in the extreme case be sufficient to locally exceed the critical temperature of the superconducting material, causing it to locally become normally-conducting, which would lead inevitably to a quench if the current feed continues.
In order to prevent movement of the conductor, it is thus necessary to embed the coil conductor without error in the sealing compound. This embedding is also frequently called “impregnating”. A filler-free, relatively low-viscosity sealing means (impregnating resin), normally based on epoxy resin, is used for this purpose as a result of the very tight conductor winding and the very narrow gaps between the wound individual conductors. However, it is problematical that the tear resistance in such sealing compounds drops with decreasing temperature, meaning that the danger of the occurrence of stress cracks increases in wound superconducting coils specifically in operation when they are thus cooled to the extremely low temperature as cited above (thus typically <80 K).
From DE 10 2007 008 122 A1 an arrangement for cooling a gradient coil is known in which cooling tubes for coolant transport are arranged to dissipate heat from coil layers of the gradient coil. For electrical insulation, insulating plates are arranged both between the coil layers and between the coil layers and the respective cooling hoses. The insulating plates include textile layers (known as “prepregs”) that are impregnated with a reaction resin. The resin contains a filler material that has good heat conduction capability. Nanoparticles can also be provided as an additional filler. Remaining interstices at intersection points of the fibers of the textile layers are also filled by the fillers.
WO 2006/027139 A1 describes a polymer composition that contains at least one resin selected from the group consisting of vinyl ester resins, unsaturated polyester resins, acrylates and methacrylates, at least one copolymer with groups reactive to resins and a glass transition temperature Tg of −20° or less, and nanoparticles with an average particle size dmax of 5-150 nm (measured by means of small-angle neutron scattering, SANS). The copolymer forms rubber domains in the cured state.